1. Field of the Invention
The invention concerns a grating structure by high dispersion volume holography. It applies in particular in pulse compression or stretching devices, e.g. for the production of ultrashort or very high energy laser pulses. It also applies in the field of optical telecommunications, e.g. for wavelength division multiplexing devices or wave trap devices.
2. Discussion of the Background
In the fields of physics and chemistry, numerous analyses and many types of processing require the use of very short, high-energy laser pulses. For example, very powerful laser pulses are required in the field of plasma physics. Pulses with very high peak power are also useful in the field of machining materials, since they result in cleaner and more accurate contours, by reducing the heating of the materials machined.
To date, the production of very high peak power laser pulses (Tera or Peta Watts) uses the CPA (Chirped Pulse Amplification) technique. This technique consists of:                stretching a low energy femtosecond pulse to make it nanosecond,        amplifying its energy,        compressing the high energy pulse obtained to make it femtosecond.        
We therefore obtain an ultrashort, very high energy pulse, i.e. of very high peak power.
Traditionally, the pulse stretching and compression operations are carried out using diffraction gratings, according to an arrangement recommended by Treacy (IEEE Journal of Quantum Electronics, vol QE-5 No. 9, September, 1969 p. 454–458) of which a simplified diagram is shown in FIG. 1. The arrangement represented in FIG. 1, for example a pulse compressor, comprises in particular two diffraction gratings 11, 12 arranged parallel to each other. The first grating 11 receives at an angle of incidence θ to the normal, an incident laser pulse FIN (represented with a single arrow) whose wavelength components are variable around a mean central wavelength λ0. The diffracted beam is diffracted in turn by the second parallel grating 12, thereby forming a beam parallel to the incident beam. This beam is reflected by reflecting means 13 to the second grating 12 (beam indicated with a double arrow) and follows a return path identical to the outgoing path, forming, at the output of the first grating 11, the output beam FOUT. The property of diffraction gratings to diffract the components with different wavelengths by a different angle depending on the wavelength is used. Thus, FIG. 1 represents the optical path for two components with wavelengths λ (dotted line) and λ′ (solid line), where λ′ is greater than λ. The optical path and therefore the travel time taken by the component at λ′ is greater than that of the component at λ, so that the pulse energy is concentrated in the output beam FOUT in a very short period for all wavelengths.
The diffraction gratings used until now are engraved gratings, i.e. gratings with lines engraved on the surface at a regular pitch. However, they do not offer complete satisfaction. A disadvantage with engraved gratings is that their throughput, approximately 90%, only results in a low compression efficiency, approximately 65%. Another disadvantage of these engraved gratings is that they display poor resistance to the laser flow. For example, for a pulse of wavelength 1053 nm and a duration of 250 femtoseconds, the gold engraved gratings display a resistance to laser flow of less than 1 J/cm2.
Consequently, users equipped with installations producing considerable energy are unable, in practice, to fully benefit from all the power they would expect from an efficient compressor and/or stretcher.